I wouldn't trust it. I've ordered a few batches of board games through the works with this and all have bounced back. Got emails from quidco saying the following:
Unfortunately, on this occasion your claim has been declined because the cashback from your sale will not be paid to us by the retailer. In order to pay us, they have to recognise us as having been the party who referred the sale to them, and in this case they do not. To explain, cashback is actually a referral commission that we receive from the retailer, for referring you to them and making a purchase. Instead of keeping it we pass it on to you, however we do have to receive it from the retailer in order to pass this on.
Some people might have better luck...